1. Field of the Invention
The present invention relates to an emulsion to be used for forming a vibration damper. The emulsion of the present invention is applied to a member such as the chassis of a vehicle which is expected to manifest a vibration damping property.
2. Description of Related Art
For the purpose of keeping a place prone to transmission of vibration in quiescence, a vibration damper which absorbs the energy of sound is utilized. By the vibration damper which is disposed as on the chassis of an automobile, for example, the vibration transmitted from the road surface and the engine is reduced and the environment inside the cabin is consequently improved.
As the material to be used for vibration damper, the rubber-based latex and the acrylic copolymer have been proposed. In equipment such as the vehicle which is used at temperature varied in a wide range from below the freezing point to several tens of degrees centigrade, the function of the vibration damper is required to be manifested in the broad temperature range. As a conventional technique for complying with this requirement, the emulsion for vibration damper which contains a core-shell type particle having a core part formed of an acrylic copolymer and a shell part formed of an acryl copolymer, which covers the core part, has been proposed (refer to JP-A-S53-78234 and EP 507634, for example). The core-shell type particles possess an excellent vibration damping property in a wide range of temperature as compared with an emulsion using acryl copolymer alone or an emulsion using two or more species of acryl copolymer as blended.
In the formation of a vibration damper by the use of an emulsion containing polymer particles, however, the thermally drying property of the coating film has posed a problem.
In order for the coating film to function as a vibration damper, it is required to have a fixed thickness. When the coating film of a large thickness is to be dried, however, since the drying of this coating film tends to proceed from the surface inwardly, the part of the coating film near the surface is hardened while the inner part of the coating film is still retaining moisture. This trend is particularly conspicuous when the coating film is formed by using an emulsion because the coating film formed of the emulsion, when the amount of water surrounding the particles decreases, is immediately fused and allowed to form the film.
When the water in the coating film is vaporized after the part of the coating film near the surface has been hardened, the problem that the already hardened part of the coating film near the surface inflates toward the outside of the coating film and the problem that the coating film sustains a crack can occur. When the coating film inflates or sustains a crack, the vibration damper will suffer the characteristic property thereof to be greatly degraded. This drawback will eventually prevent any improvement attained in such core-shell type particles which are contained in the emulsion from manifesting its characteristics fully satisfactorily.
As another conventional means for enabling the vibration damping property to be manifested in abroad temperature range, the method of mixing two or more kinds of polymers having different glass transition points has been proposed. For example, a mixture of aqueous dispersions of two or more kinds of polymers of different glass transition points, namely a polymer of a low glass transition point and a polymer of a high glass transition point, specifically a mixture in which the polymer of a low glass transition point has a glass transition point in the range of −20–10° C. and the polymer of a high glass transition point has a glass transition point in the range of 20–50° C., the difference between the glass transition points of the two polymers falls in the range of 30–50° C., and a compatibilizing agent is incorporated, has been proposed as a water-based emulsion for vibration damper (JP-A-2001-152028).
The water-based emulsion of this construction for vibration damper realizes the broadening of the temperature peak of the vibration damping property thereof by using the mixture of the aqueous dispersions of mutually incompatible polymers and further, in view of the incompatibility of the polymers, by further incorporating in the mixture a solvent and/or a cohesiveness imparting resin which possesses the quality of gradually volatilizing at room temperature as a compatibilizing agent in an amount of 1–70 parts by weight based on 100 parts by weight of the polymer component.
Since the emulsion essentially contains the compatibilizing agent, however, it entails the problem that the compatibilizing agent will persist in the coating film and degrade the vibration damping property of the emulsion.